An acrylic resin represented by PMMA is excellent in an optical property and was applied to various kinds of optical materials as an optical isotopic material having a high light transmittance, a low birefringence, and a small retardation. However, in these days, with advancement of flat displays such as a liquid crystal display device, a plasma display, and an organic EL display and advancement of infrared sensors, light guides, and the like, an optical material has been recently required to have not only excellent transparency and heat resistance but also high optical isotropy, i.e., so-called a low birefringence material has been required.
A known example of a thermoplastic resin having transparency and heat resistance is a lactone-ring-containing polymer obtained by carrying out a lactone cyclization condensation reaction with respect to a polymer having hydroxyl group and ester group in its molecular chain (see Patent Document 1 and Patent Document 2 for example).    [Patent Document 1]    Japanese Unexamined Patent Publication Tokukai 2000-230016 (Aug. 12, 2000)    [Patent Document 2]    Japanese Unexamined Patent Publication Tokukai 2000-302815 (Oct. 31, 2000)